IP Address
Every user on a TCP/IP system has to be assigned a unique identifier called an IP address. Because there are a limited number of acceptable addresses, most ISPs now assign an address from the pool they're permitted to use randomly to each user as the user calls in to connect to the system--dynamic IP addressing There was a time, though, when most ISPs permanently assigned one IP address to each of their users (fixed IP addressing), and you could identify that user by checking the headers of his newsgroup and email messages for that address. For instance, if your IP address was once 168.125.39.221 and you could find that in any post you made during that period.